The present invention relates to an image forming apparatus in which an recording material accommodating means for accommodating recording materials, an image forming means for forming an image onto the recording material, a conveyance section to convey the recording material from the recording material accommodating means to the image forming means, and a reversal and sheet re-feeding means having a sheet re-feeding reversal section and a sheet re-feeding conveyance section for conducting image formation on two-sides of the recording material are provided, and by which images can be formed on two-sides of the transfer material.
Conventionally, a following image forming apparatus in which an recording material accommodating means for accommodating a recording material, an image forming means for forming an image onto the recording material, a conveyance section to convey the recording material from the recording material accommodating means to the image forming means, and a reversal and sheet re-feeding means having a sheet re-feeding reversal section and a sheet re-feeding conveyance section for conducting image formation on two-sides of the recording material are provided, and by which images are formed on two-sides of the transfer material, is widely spread to a copier, printer, facsimile device, and the like. In such the image forming apparatus, for example, an image forming apparatus for forming an image by the electrophotographic method has the following structure to form two-sided images: an image forming means such as a charging means, image writing means and developing means, is arranged around an image carrier; by using the image forming means, after the image carrier is uniformly charged by the charging means, an electrostatic latent image is formed on the image carrier by the image writing (image exposure) of the image writing means, and the latent image is developed by developers involved in the developing means and formed into a visible toner image; the toner image is transferred onto one-side of the recording sheet conveyed from the recording material accommodating means through the conveyance section, and the toner image is formed on the recording sheet; after the toner image on the one-side of the recording sheet is fixed by the fixing means, the recording sheet is conveyed to the reversal and sheet re-feeding means through a sheet delivery and reversal means, and conveyed to the image forming means through the sheet re-feeding reversal section, a sheet re-feeding conveyance section and conveyance section again of the reversal and sheet re-feeding means; the toner image is formed on the other side of the recording material by the image forming means; the toner image on the other side of the recording material is fixed; and thereby, two-sided images are formed.
Such the image forming apparatus has, generally, a register roller to temporarily stop the recording material in the vicinity of the front of the image forming means, in order to adjust the timing of the recording material conveyed from the recording material accommodating means through the conveyance section, and the recording material conveyed through the sheet re-feeding reversal section structured by the delivery sheet and reversal means and the reversal and sheet re-feeding means to form the two-sided images, the sheet re-feeding conveyance section and the again conveyance section, to the image formation by the image forming means. The register roller is structured such that the recording material is temporarily pushed to the register roller and stopped so that the conveyance of the recording sheet is in timed relationship with the image formation. Further, the register roller is structured such that, after the register roller temporarily stops the recording material, the register roller forms a loop on the recording sheet, and thereby, the skew of the leading edge of the recording material can be corrected.
As described above, by a register roller provided in the vicinity of the front of the image forming means, the timing of the recording material to the image formation is adjusted, and further, the skew of the leading edge of the conveyed recording material is corrected, and thereby, the two-sided images are formed on a correct position of the recording material.
However, recently, in the smaller image forming apparatus, an increase of copy sheets (an increase of productivity) per unit time is required, and an increase of speed is intended. Accordingly, an improvement of each portion of the image forming apparatus or the more exact control of the sheet conveyance is tried, however, there are problems such as the cost-up of the image forming apparatus or the lowering of the reliability by the complexity of the control, and the satisfied improvement can not be attained.
That is, in the image forming apparatus to form the two-sided images, after the fixing, the conveyance path of the recording material to the reversal and sheet re-feeding means in which the recording material is reversed by the sheet re-feeding reversal section and re-fed by the sheet re-feeding conveyance section through the sheet delivery and reversal means for the two-sided image formation, is long, and by the cause of an increase of the frictional resistance between the recording material and the conveyance guide, or an increase of slippage between the recording material and the conveyance roller, a large sheet skew is generated at the conveyance path of the sheet delivery and reversal means before the recording material is arrived at the register roller, or the reversal and sheet re-feeding means, and the sheet skew cannot be corrected by the correction of the sheet skew of the register roller provided in the vicinity of the front of the image forming means, and there occurs a problem that reverse toner image can not be formed at the correct position on the recording material. This sheet skew becomes conspicuous, the more increase of the speed of the apparatus is intended, and the longer is the conveyance path at the sheet delivery and reversal means or the reversal and sheet re-feeding means.